This is the first time, i gonna make my own wow addon, and i am a bit newbie in this. Searching for something Event / API, what offers this ability, but nothing found (http://wowprogramming.com/docs however this site not offers really precise info).

For example Carbonite addon can alarm me, if an enemy player appears in range. Unfortunately that addon is *huge*. Explore info from that is about an "Impossible Mission".